Talks between British Airways and Unite broke down after the union announced strike dates. Photograph: AP

Gordon Brown has intervened personally in the British Airways industrial dispute in an attempt to avert a walkout by 12,000 cabin crew next weekend.

The prime minister called the joint general secretary of the Unite trade union, Tony Woodley, to discuss potential solutions to the looming three-day walkout, according to sources close to the dispute. Brown's private communications this weekend are believed to have taken a less aggressive tone to the one used todayby the transport secretary, Lord Adonis, who launched a broadside at one of the Labour party's biggest donors.

The Conservatives will this week launch a new attack on Labour's ties with the unions in an attempt to portray the party as going back to its Labour roots. Eric Pickles, the Tory party chairman, wrote to Brown tonight calling on him to condemn the strike and suspend Labour's financial relationship with Unite until it is settled. "In the face of this reckless [strike] action, you are giving the impression of siding with the union," he said.

Adonis admonished Unite for threatening a "totally unjustified" series of strikes, starting with a three-day action from 20 March and a further four-day walkout from 27 March – days before the expected announcement of a general election on 6 May. Unite has also reserved the right to strike from 14 April onwards, in the middle of an election battle, if there is still no agreement in a row over staffing cuts on flights. Adonis told BBC1's Andrew Marr Show: "I absolutely deplore the strike, it is not only the damage it is going to do passengers and the inconvenience it's going to cause – which is quite disproportionate to the issues at stake – but also the threat it poses to the future of one of our great companies in this country."

Hopes of a resolution to the dispute plummeted on Friday when Unite announced strike dates, triggering the withdrawal of a formal peace offer by BA. Little more than an hour before BA withdrew the offer, Unite had said it would put the proposal to its members in a consultative ballot, while simultaneously setting strike dates for eight days later in case cabin crew rejected the deal.

Observers say the offer, which included a partial repeal of staffing cuts and a three-year pay agreement, might well have received consideration from a membership that had voted 81% in favour of a strike weeks earlier.

However, the BA chief executive, Willie Walsh, said he withdrew the offer because it was conditional on Unite and its cabin crew branch, Bassa, not setting strike dates.

Unite accused Adonis of being "badly informed" and said BA had lost an opportunity to end the dispute. "Lord Adonis should publicly urge management to put that offer back on the table. Should they do so, there is still a possibility of peace," said a Unite spokesman. A Bassa source said some members are now seeking to exclude Labour party donations from their monthly dues.

A Downing Street source refused to give a running commentary on the prime minister's involvement in the strike, but insisted that Brown had already condemned the action as "unacceptable". The source said: "The PM has condemned it, Andrew has condemned it. The whole government is clear that we want both sides back at the table and talking."

Meanwhile preparations for the strikes are under way on both sides. Walsh said BA will announce a revised schedule for flights during the strike tomorrow, having pledged todayto operate up to seven out of 10 of BA's daily services during a walkout.

BA would normally expect to carry more than 500,000 passengers over the seven days of strike action. The airline is hoping to overcome the absence of staff with 1,000 volunteer flight attendants drawn from the ranks of its non-cabin crew and 23 aircraft leased from other operators, which come complete with crew.

However, the temporary workforce will have to pass picket lines at Heathrow airport organised by Unite. Bassa representatives will meet police at Heathrow tomorrowto discuss where they are legally allowed to place picket lines – the first since a cabin crew walkout in 1997.

The dispute is rooted in BA's attempts to cut its cabin crew budget in the face of record losses, including a pre-tax loss of £401m last year. It has achieved savings of £62.5m by unilaterally reducing cabin crew numbers on flights through a voluntary redundancy programme and offering staff part-time contracts. Unite says the move was antagonistic because it was imposed and damaged service standards.

Instead Unite has tabled a counter-proposal that it claims would save the same amount of money while restoring the majority of staffing cuts. Walsh has criticised the offer, saying both sides are "not close at all" to a deal.

The Civil Aviation Authority also warns of a prolonged downturn for the UK airline industry tomorrowas it confirms that last year saw the steepest fall in air passenger numbers since the second world war. UK airports handled 218 million passengers during 2009, a fall of 17 million on 2008, said the aviation watchdog.

"Passenger numbers are back to the level they were six years ago … and it could be a number of years before they reach their peak again," said Harry Bush, director of economic regulation at the CAA.